1887 ] Microscopy. 403 
onic cells. The argentea may be very easily separated in large 
sheets by “ora for four or five days in bichromate of potash 
of one per i 
Sulphuric aed is a most valuable macerating as well as pre- 
servative reagent. In weak solutions (forty drops to fifty grammes) 
entire molluscs, without the shell, have been kept in a perfect 
state of preservation for more than six months. For cilia and 
merve-endings it is exceptionally good. 
The eyes of Arca and Pectunculus may be macerated either 
weak solution which is allowed to act for a longer period. 
Chromic acid, one-fifth per cent. for ten or twelve ays gave 
most of the preparations from which the drawings of the nerve- 
endings were made. A few drops of acetic and osmic acid added 
epithelium of marine molluscs. Such preparations led to the 
discovery of the very delicate outward continuations of the pig- 
mented cover-cells in the compound eyes of Arca. 
. ARTHROPODS.—In order to demonstrate the presence of 
the corneal hypodermis in the faceted Arthropod eye, and the 
connection of the so-called “ rhabdom” with the crystalline cone 
cells, it is necessary to resort to maceration. In most cases it is 
hardly possible to determine these important points by means of 
sections alone 
The ommateum of fresh eyes, treated for twenty-four hours 
or more with weak sulphuric or chromic acid, or in Miller’s 
fluid, may be daily, removed, leaving the corneal facets with the 
underlying hypodermis uninjured. Surface views of the cornea 
epared in this way show the number and arrangement of the 
corneal cells on each facet. In macerating the cells of the om- 
mateum it is not possible to give any definite directions, for the 
results vary greatly with different eyes, and it is also necessary 
to modify the treatment according to the special point to be de- 
termined. It is as essential to isolate the individual cells as it is 
to study cross and longitudinal sections of the pigmented eyes. 
In determining the number and arrangement of the cells and the 
distribution of the pigment the latter method is indispensable ; 
it should not be replaced by the study of depigmented sections, 
which should be resorted to in special cases only. 
In fixing the tissues of the eye, it is not sufficient to place the 
detached head in the hardening fluid; the antenaz and mouth 
it is oo to do so with safety, the head should be cut open 
and all unnecessary tissue and hard parts removed. With 
Sr i material, one often finds individuals in which it is 
possible to pah: m the %ardened tissues of the eye 
VOL, XXI.—NO 
